Loss of Water Pressure in New Shower Install

Hi. I just installed a stand-up shower in a new bathroom. However, when I tested the shower, the water pressure was only a trickle. So, I adjusted the screws on the pipe to increase the water pressure, and I was happy to see the water flow. I left the reno that evening, and my friends all took showers without problem (2 showers). Then, the next day, the shower was no longer working (same trickle problem). Any ideas

Most likely problem is debris at the showerhead or debris in the valve. I'll bet somewhere in the instructions for installing that valve, it recommended flushing out the lines. What brand and model did you install?
Deb
The Pipewench

It's a Rona/Revy brand (no idea on the model number). I've been through the instructions several times, and I don't recall seeing information about flushing the lines

I wondered myself about debris, but I didn't solder (it's all crimp joints and plastic tube), and I can't see any place for it to plug? As of last night, the hot water is trickling, and the cold is completely off. My plan this evening is to start dismantling and see if I can find the place where the problem lies.

The screws are located on the faucet itself, at the location of the water intakes.

Thanks for the guided steps. That's the action I was contemplating for this evening (with the exception of the shower head because I'm not getting any cold water, so that hardware is already ruled out).

Ok, last night I went over to fix the shower, and it's already magically WORKING? No longer broken? Very weird. I guess I'll wait it out and see what happens in the upcoming weeks. I just find it weird, because it sounds like debris, but where could debris deposit in such a way that it would completely block cold water, and only allow a trickle of hot water through.

Debris flakes away from the internal walls of water piping any time the water is shut off and turned back on again. It is very very easy for debris to completely shut off the flow of water to a faucet or valve. (One of the most common remedies on this forum for "I have no water out of my faucet " is "Remove and clean the aerator").
Jimbo has given you the right steps in the right order. However, since you have integral stops, you can shut the water off there and remove the workings of the valve. If this is a pressure balance valve, you could have some kind of balancing spool that is sticking or defective.
Deb
The Pipewench